function Hv = H( alpha, beta )

    N=4;

    k=[cos(alpha)*cos(beta);
       cos(alpha)*sin(beta);
       sin(alpha)];

    r{1}=[0.5 -0.5 0];
    r{2}=[-0.5 -0.5 0];
    r{3}=[-0.5 0.5 0];
    r{4}=[0.5 0.5 0];

    phi=nan(4,1);

    for i = 1:N
        phi(i)=2*pi*r{i}*k;
    end

    Hv=exp(1i*phi);
end